1. Field of the Invention
The present invention relates to a hydraulic control apparatus for controlling a hydraulically operated power transmitting system used for a motor vehicle, which includes a continuously variable transmission, a reversing gear device and an auxiliary transmission.
2. Discussion of the Related Art
An example of a vehicle power transmitting system including a continuously variable transmission whose speed ratio is continuously variable is disclosed in laid-open Publication No. 61-2958 of unexamined Japanese Patent Application. The power transmitting system disclosed in this publication also includes an auxiliary transmission having a plurality of forward gear positions for increasing the range of speed ratio of the transmitting system. To reduce the whole size of the power transmitting system, it is considered to install the auxiliary transmission apart from a reversing gear device for selecting forward or reverse running of the vehicle, as disclosed in Japanese Patent Application No. 3-74468 filed on Mar. 14, 1991 in the name of the assignee of the present application. More specifically, the power transmitting system of this type includes: the continuously variable transmission; the reversing gear device which is rotated in opposite directions to permit forward and reverse running of the vehicle, and which includes forward and reverse frictionally coupling devices that are hydraulically operated depending upon a currently selected position of a shift lever of the vehicle; and the auxiliary transmission operable between a low-gear position which is established by engagement of a low-gear frictionally coupling device and a one-way clutch, and a high-gear position which is established by engagement of a high-gear frictionally coupling device.
A hydraulic control apparatus for controlling the above type of the power transmitting system is required to smoothly shift the auxiliary transmission from the low-gear position to the high-gear position, by engaging the high-gear frictionally coupling device while the low-gear frictionally coupling device is kept engaged, by utilizing engagement and disengagement of the one-way clutch. During a given period of time after the shift lever is operated from a neutral position to one of forward-drive positions, or while the shift lever is being operated to the neutral position, the hydraulic control apparatus is required to release the low-gear frictionally coupling device and engage the high-gear frictionally coupling device so as to perform an anti-squat control for lowing a driving torque transmitted through the transmission. Further, upon completion of this anti-squat control, the control apparatus is required to engage the low-gear frictionally coupling device and release the high-gear frictionally coupling device to establish the low-gear position of the auxiliary transmission. Since the low-gear and high-gear frictionally coupling devices need to be controlled independently of each other under the above situation, it is difficult to control the auxiliary transmission by using a common pilot pressure generated by a single solenoid-operated valve, as in a hydraulic control apparatus disclosed in the above-identified publication No. 61-2958. Thus, the known hydraulic control apparatus undesirably requires two solenoid-operated valves for controlling the respective low-gear and high-gear frictionally coupling devices.
The hydraulic control apparatus for the power transmitting system of the above type has another problem. Namely, when the shift lever is operated from a forward-drive position to a reverse-drive position while the auxiliary transmission is placed in the low-gear position, for example, the reverse frictionally coupling device operates to rotate the reversing gear device in the reverse direction to permit reverse running of the vehicle, and the high-gear frictionally coupling device operates to shift the auxiliary transmission from the low-gear position to the high-gear position.
When the shift lever is operated from the forward-drive position to the reverse-drive position while the auxiliary transmission is placed in the low-gear position, the reverse frictionally coupling device is engaged after the reversing gear device is held in a neutral position for some time upon releasing of the forward frictionally coupling device. At the same time, the high-gear frictionally coupling device is engaged after the auxiliary transmission is held in a neutral position for some time upon releasing of the low-gear frictionally coupling device. Thus, the reverse frictionally coupling device and high-gear frictionally coupling device are engaged in substantially the same period of time while the power transmitting line of the system is disconnected at the reverse gear device and auxiliary transmission. The engagement of each of the reverse and high-gear frictionally coupling devices is accomplished after a given period of engaging time in which the pressure for engaging the device is slowly increased by an accumulator, for example. Generally, the high-gear frictionally coupling device is provided with a relatively small torque capacity due to the function of the corresponding accumulator, so as to alleviate a shift shock induced upon shifting of the auxiliary transmission from the low-gear position to the high-gear position during running of the vehicle. Accordingly, if the reverse and high-gear frictionally coupling devices are substantially concurrently engaged, the engagement of the reverse frictionally coupling device is first completed, which is followed by completion of the engagement of the high-gear frictionally coupling device, due to the function of the respective accumulators. In this case, however, there may arise a double shift shock due to the engagements of the reverse and high-gear frictionally coupling devices, owing to the relatively small torque capacity of the high-gear frictionally coupling device during its engaging action, which makes it impossible to sufficiently absorb a shock induced upon establishment of the reverse gear position of the reversing gear device. The small torque capacity also causes slipping of the high-gear frictionally coupling device, resulting in reduced durability of this device.
In the above-described hydraulic control apparatus for the power transmitting system including the reversing gear device and the auxiliary transmission, the auxiliary transmission is placed in the low-gear position by engagement of the low-gear frictionally coupling device and one-way clutch which are disposed in series, and in the high-gear position by engagement of the high-gear frictionally coupling device. Where the low-gear and high-gear frictionally coupling devices are both placed in their released positions, the auxiliary transmission is placed in its neutral position for disconnecting the power transmitting line between the engine and drive wheels of the vehicle. For controlling the auxiliary transmission so that only the high-gear frictionally coupling device is engaged, or so that one of the high-gear and low-gear frictionally coupling devices is released while the other is engaged, the hydraulic control apparatus needs two control valves for the high-gear and low-gear frictionally coupling devices, respectively, and is required to perform a subtle timing control over these two control valves for engagements of the two frictionally coupling devices. This timing control is difficult to perform owing to a difference in the engaging characteristics between the individual frictionally coupling devices, a chronological change of the engaging characteristics, and variation of the engaging characteristics with the temperature of the working fluid. Namely, when the auxiliary transmission is shifted by engaging one of the low-gear and high-gear frictionally coupling devices and releasing the other, the vehicle will suffer from racing of the engine if the engagement of the one device after the releasing of the other takes place too late, and from a shift shock due to locking of the auxiliary transmission if the engagement of the one device after the releasing of the other takes place too early.